The catalytic hydrogenation of carbon monoxide to produce light gases, liquids and waxes, ranging from methane to heavy hydrocarbons (C80 and higher) in addition to oxygenated hydrocarbons, is typically referred to as Fischer-Tropsch or FT synthesis. Traditional FT processes primarily produce a high weight percent FT wax (C25 and higher) from the catalytic conversion process. These FT waxes are then hydro-cracked and/or further processed to produce diesel, naphtha, and other fractions. During this hydro-cracking process, light hydrocarbons are also produced, which may require additional upgrading to produce viable products. These processes are well known and described in the art.
As indicated above, the costs associated with the production of synthesis gas for use in an FT process, such as liquid fuel production, represent a significant portion of the total cost of the plant and the quality characteristics of the synthesis gas is critical to the efficient operation of the plant. The synthesis gas used in the FT synthesis is typically characterized by the hydrogen to carbon monoxide ratio (H2:CO). A H2:CO ratio of from about 1.8 to about 2.1 defines the desired ratio of synthesis gas used in many gas to liquids production process.
Synthesis gas containing hydrogen and carbon monoxide is produced for a variety of industrial applications. Conventionally, the synthesis gas is produced in a steam methane reforming (SMR) process using a fired reformer in which natural gas and steam are reformed in nickel catalyst containing reformer tubes at high temperatures (e.g., 850° C. to 1000° C.) and moderate pressures (e.g., 16 to 30 bar). The endothermic heating requirements for steam methane reforming reactions occurring within the reformer tubes are provided by burners firing into the furnace that are fueled by part of the natural gas. In order to increase the hydrogen content of the synthesis gas produced by the steam methane reforming (SMR) process, the synthesis gas can be subjected to water-gas shift reactions to react residual steam in the synthesis gas with the carbon monoxide.
A well-established alternative to steam methane reforming is the non-catalytic partial oxidation process (POx) whereby a sub-stoichiometric amount of oxygen is allowed to react with the natural gas feed creating steam and carbon dioxide at high temperatures. The high temperature residual methane is reformed through catalytic reactions with the high temperature steam and carbon dioxide. Yet another attractive alternative process for producing synthesis gas is the auto-thermal reformer (ATR) process which uses oxidation to produce heat with a catalyst to permit reforming to occur at lower temperatures than the POx process. However, similar to the POx process, the ATR process requires oxygen to partially oxidize natural gas in a burner to provide heat, as well as high temperature carbon dioxide and steam to reform the residual methane. Normally some steam needs to be added to the natural gas to control carbon formation on the catalyst. However, both the ATR as well as POx processes require an air separation unit (ASU) to produce high-pressure oxygen, which adds complexity as well as capital and operating cost to the overall process.
When the feedstock contains significant amounts of heavy hydrocarbons, the SMR and ATR processes are typically preceded by a pre-reforming step. Pre-reforming is a catalyst based process for converting higher hydrocarbons to methane, hydrogen, carbon monoxide and carbon dioxide. The reactions involved in pre-reforming are typically endothermic. Most pre-reformers operate adiabatically, and thus the pre-reformed feedstock typically leaves at a much lower temperature than the feedstock entering the pre-reformer. Another process that will be discussed in this invention is the secondary reforming process, which is essentially an ATR type process that is fed the product from a steam methane reforming process. Thus, the feed to a secondary reforming process is primarily synthesis gas from steam methane reforming. Depending on the end application, some natural gas may bypass the SMR process and be directly introduced into the secondary reforming step. Also, when a SMR process is followed by a secondary reforming process, the SMR may operate at a lower temperature, e.g. 650° C. to 825° C. versus 850° C. to 1000° C.
As can be appreciated, the conventional methods of producing a synthesis gas such as have been discussed above are expensive and require complex installations. To overcome the complexity and expense of such installations it has been proposed to generate the synthesis gas within reactors that utilize an oxygen transport membrane to supply oxygen and thereby generate the heat necessary to support endothermic heating requirements of the reforming reactions. A typical oxygen transport membrane has a dense layer that, while being impervious to air or other oxygen containing gas will transport oxygen ions when subjected to an elevated operational temperature and a difference in oxygen partial pressure across the membrane.
Examples of oxygen transport membrane based reforming systems used in the production of synthesis gas can be found in U.S. Pat. Nos. 6,048,472; 6,110,979; 6,114,400; 6,296,686; and 7,261,751. There is an operational problem with some or all of these oxygen transport membrane based systems because such oxygen transport membranes need to operate at high temperatures of around 900° C. to 1100° C. Where hydrocarbons such as methane and higher order hydrocarbons are subjected to such high temperatures within the oxygen transport membrane, excessive carbon formation occurs, especially at high pressures and low steam to carbon ratios. The carbon formation problems are particularly severe in the above-identified prior art oxygen transport membrane based systems. A different approach to using an oxygen transport membrane based reforming system in the production of synthesis gas is disclosed in U.S. Pat. No. 8,349,214 which provides a oxygen transport membrane based reforming system that uses hydrogen and carbon monoxide as part of the reactant gas feed to the oxygen transport membrane tubes and minimizes the hydrocarbon content of the feed entering the permeate side of the oxygen transport membrane tubes. Excess heat generated within the oxygen transport membrane tubes is transported mainly by radiation to the reforming tubes made of conventional materials. Use of low hydrocarbon content high hydrogen and carbon monoxide feed to the oxygen transport membrane tubes addresses many of the highlighted problems with the earlier oxygen transport membrane systems.
There is a continuing need to improve the efficiency and cost-effectiveness of production of liquid hydrocarbon products from a Fischer-Tropsch process. Accordingly, there is a specific need to identify and develop advanced technologies that will improve the efficiency and reduce the cost of producing synthesis gas for use in applications for producing liquid fuels, as well as improving or customizing the characteristics of synthesis gas for such applications.